Integrin, a cell surface glycoprotein, is an adhesion molecule that functions mainly as a receptor for cell adhesion to extracellular matrices (collagen, laminin and the like) and members of the immunoglobulin family (ICAM-1, VCAM-1 and the like), and mediates signal transduction from extracellular matrices. Thereby, cells receive signals from the extracellular matrices, and differentiation, proliferation, cell death and the like are induced. Integrin is a heterodimer consisting of the two subunits α chain and β chain; there are different α chains and β chains occurring in a wide variety of combinations, and there are 24 members of the integrin superfamily. Integrin-knockout mice are fatal or diseased irrespective of which subunit is lacked, suggesting that individual integrins are necessary for the maintenance of life. Therefore, integrin, which transmits information on ambient conditions to cells to stimulate their responses, are thought to function in all situations of biological phenomena, and to mediate a broad range of pathologic conditions.
As such, integrin is indispensable to the survival of organisms, and is thought to play roles even in diseased states; some cases have been reported in which their inhibition helps improve pathologic conditions. For example, an inhibitor of platelet-specific integrin αIIbβ3 has been approved as a therapeutic drug for PCTA restenosis known as abciximab (trade name: ReoPro; Eli Lilly). Natalizumab (trade name: Antegren; ELAN Company), an α4β1 (VLA4) inhibitor, has been approved as a therapeutic drug for multiple sclerosis. The αvβ3 inhibitor Vitaxin (MEDIMMUNE Company) is under development in clinical studies for its neovascularization inhibitory action, osteoclast activation inhibitory action and the like.
Integrin α9β1 is expressed in macrophages, NKT cells, dendritic cells, and neutrophils, and reportedly plays important roles in the infiltration and adhesion of these inflammatory cells, bone resorption and the like. Recently, it has been reported that integrin α9β1 is involved in osteoclast formation, and its involvement in bone destruction has been suggested (Non-patent Document 1). Known ligands thereof include truncated osteopontin (N-terminal OPN), VCAM-1, Tenascin-C and the like. Clinically, significantly elevated levels of integrin α9β1 have been observed in the synovial tissues of patients with rheumatoid arthritis (Non-patent Document 2).
Therefore, a monoclonal antibody that binds specifically to α9 integrin protein to act to inhibit α9 integrin-dependent cell adhesion, if developed, would be useful in the diagnosis, prevention or treatment of various diseases involved by α9 integrin in their pathogenesis.
Antibodies that have been reported to exhibit function inhibitory action on human α9 integrin are the mouse monoclonal antibody Y9A2 (Non-patent Document 3), and 1K11, 24I11, 21C5 and 25B6, which are also mouse monoclonal antibodies (Patent Document 1). Although in vitro experimental results have shown that these antibodies are capable of suppressing human α9 integrin-dependent cell adhesion, they are unsuited for use in experiments for in vivo evaluations of pharmacological effects and the like because they do not exhibit cross-reactivity to mouse and rat α9 integrin.
Antibodies that have been reported to exhibit function inhibitory action on mouse α9 integrin are the hamster monoclonal antibodies 11L2B, 12C4′58, 18R18D and 55A2C (Patent Document 1). In vitro experimental results have shown that these antibodies are capable of suppressing functions of mouse α9, such as cell adhesion, and in vivo experimental results have shown that 11L2B has a therapeutic effect on hepatitis; however, their reactivity to human α9 integrin has not been confirmed, so it is impossible to apply these antibodies to the treatment or prevention of human diseases.
As the situation stands, even if an anti-human α9 integrin antibody is acquired and functionally evaluated in vitro, it is difficult to evaluate the pharmacological effect of the antibody unless it exhibits cross-reactivity to mouse or rat α9 integrin, because the available pathologic models of various inflammatory diseases are for the most part systems using a mouse or rat. Even if an anti-mouse α9 integrin antibody is acquired and pharmacologically evaluated using an in vivo pathologic model system, and is found to be therapeutically or prophylactically effective, it is impossible to apply the antibody as an antibody pharmaceutical to human pathologic conditions unless it exhibits cross-reactivity to human α9 integrin.
Provided that an anti-human α9 monoclonal antibody such as Y9A2 is developed as an antibody pharmaceutical on the basis of pharmacological effect data obtained using an anti-mouse α9 integrin antibody, a great deal of labor will be required to demonstrate equivalence of the antibody used to acquire the pharmacological data and the antibody under development. For this reason, there is a demand for, for example, an antibody that exhibits inhibitory action on function of both mouse α9 integrin and human α9 integrin; judging from the principles, however, it is difficult to acquire such an antibody when using a conventional method such as one involving mouse immunization.
Even if an anti-human α9 monoclonal antibody prepared by any technique overcoming this difficulty is developed as an antibody pharmaceutical, the antibody will be recognized and eliminated as a foreign matter because of the high immunogenicity thereof when administered to humans, as far as the antibody is an antibody derived from non-human animal. Therefore, it is difficult to use such an antibody as a therapeutic drug for a disease.
As a possible solution to this problem, a non-human-derived antibody may be humanized using a protein engineering technique; however, because a portion of the non-human-derived sequence is contained, multiple-dose administration or long-term administration can give rise to an antibody that inhibits the activity of the humanized anti-α9 integrin antibody administered to considerably weaken the effect thereof, and even can cause a serious adverse reaction. Additionally, humanization often results in decreased activity, and a humanized antibody requires a great deal of labor and cost for its construction.
As the situation stands for α9 integrin, there is almost no structural information on the steric structure, ligand binding site, neutralizing epitope and the like; such information, if obtained, is expected to open a way to research into α9 integrin and its application to medical practice, and potentially makes a great contribution.    Patent Document 1: WO 2006/075784    Non-patent Document 1: Journal of Bone and Mineral Research, 2006, 21: 1657-1665    Non-patent Document 2: The Journal of Clinical Investigation, 2005, 115: 1060-1067    Non-patent Document 3: Am. J. Respir. Cell Mol. Biol., 1996, 15: 664-672